This application is based on and claims priority from Japanese Application No. 8-225492 filed Aug. 27, 1996, No. 8-237169 filed Sep. 2, 1996, and No. 8-326944 filed Dec. 6, 1996, the contents of each of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention can be utilized for trunk transmission systems in which communication involves setting up paths semi-permanently on physical transmission lines, and it is suited for use in synchronous digital hierarchy (SDH) networks. A path is defined as a single and direct connection between a node-pair, for example, concatenated Virtual Container (VC) in SDH format, and may include Virtual Paths (VPs) having the same source-destination nodes in the trunk network.
2. Description of Related Art
In recent years, ultrahigh-speed trunk transmission networks that utilize the broadband nature of optical fiber have been introduced. In particular, as disclosed, a 10 Gbit/s transmission system has been introduced in trunk network links, as disclosed in:
Ref.1: Y. Kobayashi, Y. Sato, K. Aida, K. Hagimoto and K. Nakagawa, xe2x80x9cSDH-Based 10 Gbit/s Optical Transmission Systemxe2x80x9d, Proc. IEEE GLOBECOM 94 (San Francisco, Calif.), pp.1166 -1170, 1994
Meanwhile, asynchronous transfer mode (ATM), which supports diverse services, has been recommended by the ITU-T and the ATM Forum as the signal processing scheme for network nodes. In the ATM layer, which is positioned between the physical layer and the application layer, signals are processed in cell units. However, problems are encountered if the processing of signals carried in trunk network links at speeds in excess of 10 Gbit/s is carried out entirely in cell units. As many as several thousand virtual paths (VPs) have to be processed at each node, which means that large-scale node circuitry and more complex network management is required, as disclosed in:
Ref.2: S. Matsuoka, N. Kawase, Y. Yamabayashi and Y. Kobayashi, xe2x80x9cClassified Path Restoration Scheme With Hitless Protection Switching for Large-Capacity Trunk Transmission Networksxe2x80x9d, IEEE GLOBECOM 95, p.941-945, 1995
Given this situation, the present inventors considered that although the signal processing employed in the ATM layer can be utilized for service nodes, trunk network node processing functions such as path setup and restoration will be carried out in large-capacity direct-connected path units at the physical layer. These large-capacity direct-connected paths can have a variety of capacities, and the management of path networks can be simplified by processing in large-capacity direct-connected path units. It is also considered that time division multiplexing (TDM) will be used at the physical layer for multiplexing. In the present specification, it will generally be assumed that Synchronous Digital Hierarchy (SDH) is being used.
Meanwhile, high reliability and survivability are required in networks with ultrahigh-capacity links, as disclosed in:
Ref.3: T. -H. Wu, xe2x80x9cFibre Network Service Survivabilityxe2x80x9d, Artech House, Boston and London, 1992
In an ultrahigh-speed network, a failure in one fiber can have adverse effects on several thousands of users.
Self-healing functions are therefore being studied and introduced. Self-healing is a high-speed restoration function for network failures, and the best-known example to have been introduced is the SONET (Synchronous Optical Network) ring network in which path or line switches are provided. A self-healing ring network has the advantages of simpler equipment configuration and higher reliability. Problems of delay and the like mean that a multiple-ring configuration combining a plurality of rings is a promising approach to the design of trunk networks. However, a multiple-ring network with a self-healing function has not yet been achieved, and path setup functions such as routing and slot allocation have not yet been perfected.
Network supervision and control will now be explained. The TMN (Telecommunication Management Network) model has been standardized, and its architecture is shown in FIG. 1. In this architecture, a network element NE provided at each node is connected to a packet transfer network DCN (data communication network) via a message converter module MCM (or a mediation device MD, not shown), and an operating system OpS is connected to this packet transfer network DCN. FIG. 1 also shows a workstation WS for using operating system OpS. Each network element NE has a control section which exchanges control signals with the operating system OpS, and transfers supervisory and control information to the OpS, via the message converter module MCM (or a mediation device MD) and the packet network DCN.
However, as transmission link-capacity of the networks becomes larger, the cost of the operating system OpS in the model shown in FIG. 1, and in particular software development cost, becomes higher than that of the network elements NE, thus raising overall network costs. Moreover, with a centralized control network of the sort shown in the FIG. 1 if the system goes down at the control node, this leads to the entire network going down.
Distributed control has therefore been much studied. In distributed control, network control is performed in distributed fashion at each network node. FIG. 2 shows a distributed management network architecture in a single-ring network. With this architecture, a small-scale operating system OpS is provided at each network element NE. Distributed control of this sort is disclosed in, for example:
Ref.4: I. Cidon, I. Gopal, M. Kaplan and S. Kutten, xe2x80x9cA Distributed Control Architecture of High-Speed Networksxe2x80x9d, IEEE Transactions on Communications, Vol.43, No.5, pp.1950-1960, 1995
A distributed control network requires only a small-scale operating system provided in each network element, and gives higher reliability in relation to node failure than a centralized control network with several control nodes, as disclosed in:
Ref.5: A. E. Baratz, J. P. Gray, P. E. Green, Jr., J. M. Jaffe and D. P. Pozefsky, xe2x80x9cSNA Networks of Small Systemsxe2x80x9d, IEEE Journal on Selected Areas in Communications, Vol.SAC-3, No.3, pp.416-426, 1985
Further advantages are that a separate control network such as DCN is not needed, the network database memory held by each node can be reduced in size, and faster control is possible.
It is anticipated that in the future there will be many different kinds of multimedia services and that each kind will require different signal quality or reliability. Trunk networks will therefore have to operate and administrate multiplexed paths for each service in accordance with a diverse range of quality requirements, and do so at low cost.
However, conventional network technology handles the quality and reliability of all paths in the same manner. Consequently, the quality and reliability of a network has previously been dictated by the path which has the highest requirements, with the result that overall network cost has been high. An approach which was studied as a way of overcoming this problem, namely, to provide for different QoS (Quality of Service) classes by means of a logically configured virtual channel handler (VCH) interconnection network layer rather than at the VP layer, is described in:
Ref.6: E. Oki and N. Yamanaka, xe2x80x9cAn Optimum Logical-Design Scheme for Flexible Multi-QoS ATM Networks Guaranteeing Reliabilityxe2x80x9d, IEICE Trans. Commun., E78-B, No.7, pp. 1016-1024, 1995
However, this proposed scheme still required a high-quality VP network and lacked flexibility at the path operating level.
It is considered that future multimedia networks will require flexibility at the path level as well. In other words, such networks will simultaneously contain paths where high cost is acceptable but loss of even a single bit is not acceptable, and other paths where some deterioration of quality or reliability is acceptable but cost should be kept low.
It is an object of the present invention to provide a solution to this problem and to achieve flexibility of path operation. It is a further object of the present invention to provide a concrete implementation, in a multiple-ring architecture under a distributed control environment, of the operation of paths that have been classified in accordance with their self-healing survivability.
The present invention provides a trunk transmission network having a plurality of nodes connected via physical transmission lines, wherein a plurality of paths for transmitting information signals are set up on these physical transmission lines among the plurality of nodes. For for an information signal being transmitted from one of the plurality of nodes (a source node) to another of the plurality of nodes (a destination node), each path connects the source node and the destination node either directly or via other nodes. This trunk transmission network is characterized in that paths between source and destination node pairs are set up on the basis of a pre-classification into higher service class paths in which any loss of information occurring in the path is restored, and lower service class paths in which loss of information in the path is permitted. Each node includes means which, when that node is a source node, recognizes the service class of the information signal to be sent to the destination node and selects a path corresponding to this service class.
The higher service class is preferably further divided into a highest class (hereinafter class A) and a middle class (hereinafter class B). Class A paths employ complete diversity routing: namely, a plurality of different routes are set up for each class A path. Class B paths can be re-routed around the location of a failure when a failure has occurred in a portion of the route traversed by the path. The lowest service class path (hereinafter class C) is preferably a path which is not alternatively routed when a failure has occurred on the path.
By dividing paths between nodes of interest into three classes according to their restoration performance in the event of a failure, the transport functions required by service nodes can be secured at the path level without configuring redundant sections, thereby providing a trunk transmission network that can economize on transmission facilities. Furthermore, by managing the network using just three types of large-capacity paths, the number of paths that have to be managed in the network can be reduced, and hence the burden on the operating system can be eased.
Each of at least some of the plurality of nodes preferably has a distributed path setup means which sets up paths prior to transmission of an information signal by using a control channel to exchange control signals with other nodes. In this case, the distributed path setup means selects a route, in accordance with the required service class, from among the plurality of routes which can connect the source and destination nodes, and then sets up a path along the selected route. Path setup methods can be broadly divided into two types. In the first type, a node which wishes to transmit data takes itself as the source node and provisionally determines routes on the basis of network configuration information given in a manual. It then secures bandwidth by sending a control signal to all the nodes on the route up to the target receiving node. In the second type of path setup method, a source node uses a token protocol to send a packet to a destination node, and any intermediate nodes place a stamp in the packet indicative of whether or not the required bandwidth can be secured. This procedure enables the route to be determined and the necessary bandwidth to be secured.
The physical transmission lines are in the form of a plurality of ring networks connected together, each ring network comprising two or more nodes connected in a ring. Each ring network is connected to another ring network by means of some of the network nodes acting as bridge nodes. The distributed path setup means preferably includes means which, for a class A path, sets up two paths in mutually opposite directions, i.e., clockwise or counterclockwise, around each ring network through which the class A path passes, and which, for a class B path and a class C path, sets up a path in one direction around each ring network.
By restricting a trunk transmission network to a ring topology, setting the direction of routes is restricted to either clockwise or counter-clockwise, routing in the normal state and re-routing for path restoration, etc. after a failure can be simplified, the hardware and algorithms required for route compilation can be reduced in scale, and an economical trunk transmission network can be obtained. In addition, by arranging a plurality of ring networks in a plane and connecting these ring networks to each other using two or more nodes, high reliability, survivability and economy can be secured for large-scale trunk transmission networks, and at the same time expandability can be improved.
If the second of the methods described above is utilized for path setup, a token ring protocol constructed on a data communication channel (DCC) in the section overhead (SOH) embedded in the signal is preferably used for communication between nodes and for securing bandwidth, these functions being required for route determination under distributed control.
In other words, the means for path setup preferably comprises means which, when the node in question is a source node, gets a token which is circulating around the ring network to which that node belongs, and then sends path setup request packets in two mutually opposite directions; means which, when the node in question is a bridge node, transfers a path setup request packet that has arrived in one direction to the next ring network in the same direction; and means which, when the node in question is a destination node and the packets received from the two directions request setup of a class A path, sends back a response to these packets in two mutually opposite directions, and when the packets received from the two directions request setup of a class B or a class C path, sends back a response to one of these packets in one direction only.
For self-healing, each node preferably has means for hitlessly selecting the better quality route of the class A path including two routes for which that node is the destination node. The path setup means preferably also includes means for automatically restoring a class B path by re-routing in the event of a failure. This restoration means preferably includes means which utilizes the second setup method described above to loop back a token contained in the aforementioned control channel when a node has detected a failure in an adjacent link or node.
In other words, three classes of paths, namely class A, class B and class C are provided, and these offer three levels of reliability in terms of path restoration performance. Class A paths are accommodated by two different routes obtained by route-bifurcation at the source node, and these two routes are hitlessly switched at the destination node, with the result that when a failure occurs a class A path can be restored without the loss of even a single bit of information. A detailed description of hitless switching is given in:
Ref.7: N. Kawase, et al., xe2x80x9cHitless Frame Switching Scheme for SDH Networksxe2x80x9d, Trans. IEICE B-I (in Japanese), Vol.J78-B-I, No.12, pp.764-772, 1995
Class B paths, which have the next highest reliability, are restored by re-setup of the path by means of the same method as used for the original path setup. In the case of a class C path, the signal is not reconnected until maintenance of physical equipment has been completed. Apart from class B paths, the mechanism of self-healing is approximately the same as the one disclosed in Ref.2.
Each ring network is connected to another ring network via two or more bridge nodes. At least one node of any of the ring networks includes means for transmitting, in one direction (i.e., clockwise or counter-clockwise) of the ring network to which that node belongs, a node information collecting packet for collecting information relating to the arrangement and operating state of the nodes in that ring network and in the other ring networks, and means which terminates a node information collecting packet which has returned to the node which originally transmitted it, and which stores the information collected by that packet. Each node of each ring network includes means which writes its node ID and states in a received node information collecting packet and transfers the packet to the next node. Each node used as a bridge node includes, in addition to this node information collecting packet writing and transfer means, means for temporarily storing a node information collecting packet received from one of the two ring networks mutually connected by the bridge node in question; means which, when a right to transmit to the other of the two ring networks has been received, transfers to this other ring network the node information collecting packet stored in the aforementioned temporary storage means; means which deletes the node information collecting packet stored in the temporary storage means if no transmitting right has been obtained and a node information collecting packet from another bridge node has been received; and means for terminating a node information collecting packet which has returned to the bridge node which originally transferred it, and for write inhibiting that packet and returning it to the original ring network.
Each node and bridge node preferably includes means which, if it receives the same node information collecting packet within a predefined time, deletes this packet. A source node preferably also has means which distributes, to at least the bridge nodes of the plurality of ring networks, and if required to each node, the information collected by a node information collecting packet. Each bridge node preferably includes means which, on the basis of the information distributed from this distributing means, places a restriction on the setup of paths via that bridge node.
The inventions of Japanese Laid-Open Patent Applications Hei-3-276937 and Hei-3-217140 disclose providing differences in path priority. According to the former, the quality of a high priority path is guaranteed by sacrificing a healthy low priority path in the event of a failure in the high priority path. As opposed to this, in the present invention, both high priority paths and low priority paths include alternative paths and are set up in advance, and the setup of these paths is not changed when there is a failure. Accordingly, a low priority path is never sacrificed for a high priority path. Furthermore, Japanese Laid-Open Patent Application Hei-3-276937 discloses shared switching being carried out for purposes of path restoration, but there is no route duplication as in the highest class paths in the present invention. Japanese Laid-Open Patent Application Hei-3-276937 also discloses master nodes which perform centralized control. As opposed to this, the present invention performs distributed control.
Japanese Laid-Open Patent Application Hei-3-217140 relates to packet networks in which data transmission takes place only when data has been generated. Furthermore, it allocates one of two transmission paths to urgent data, but does not provide degrees of priority for data transfer when a failure has occurred. Moreover, master nodes perform centralized control of the data transmission paths. The present invention is entirely different from this.